Various molding methods for optical glass elements such as high precision lenses, using precisely fabricated molds made of a mold material, which can be finished to an optical surface, to which softened glass does not adhere, and which does not require grinding or polishing after press molding, have been developed recently. To obtain a required lens, the lens has to satisfy a specification such as lens thickness, outer diameter, eccentricity, as well as of surface shape precision and surface quality (smoothness on the surface). It is better for such a lens to have a good internal quality such as good refractive index and permeability as a matter of course.
To use such a molding method for glass optical elements as business, how much productivity would be gained may be a serious problem. That is, it is critical whether many glass optical elements can be produced in a short period.
One means to improve the productivity is to fabricate plural glass material pieces in parallel, and another means is to shorten the fabrication time per cycle. Various improved methods have been proposed, respectively. Shortening the fabrication time per cycle requires shortening heating and cooling periods of the molds, and therefore, various ideas are made to improve the glass material during molding and the temperature condition among the molding conditions.
For example, Japanese Unexamined Patent Publication No. 7-10,556 (hereinafter referred to as "Related art No. 1") describes a method for press-molding a glass material having a viscosity in a range of 10.sup.7 to 10.sup.9 poises with molds heated at a temperature that the glass material indicates a viscosity of 10.sup.10 to 10.sup.12 poises. Japanese Unexamined Patent Publication No. 9-12,317 (hereinafter referred to as "Related art No. 2") describes a method for press-molding a glass material having a viscosity in a range of 10.sup.5.5 to 10.sup.9 poises with molds heated at a temperature that the glass material indicates a viscosity of 10.sup.8 to 10.sup.12 poises (provided that the temperature of the molds is lower than that of the glass material).
Each method shortens the cycle time by shortening times for increasing and decreasing temperatures where the temperature of the mold is not raised more than a necessary temperature.
Moreover, in each method, to prevent the molding surface of the molds from becoming impaired, the glass material heated and softened at a place other than the molds is transferred into the molds before molding and is then molded.
Meanwhile, with such a method for molding glass optical elements, in addition to needs from production technology such as a shortened cycle time and prevention of impairments of molds, improved performance of the molded article is also sought. That is, there are also needs to provide optical elements having excellent optical property and meeting with needs.
Although the surface precision required for glass optical elements may vary depending on purpose of the glass optical elements or the like, power (Newton ring) as a scale of surface precision is required within .+-. four fringes for ordinary use, and more preferably, within .+-. two fringes, and irregularity (ASU) is required within one fringe, preferably, within 0.5 fringe.
With respect to the method set forth in an embodiment of Related art No. 1, surface precision of the molded glass optical element was measured. That is, where, after molded with pressure, a molded article is released without cooling down the article at the glass transition point or below (condition of Table 2 in Related art No. 1), it had power of 4 to 6 fringes and irregularity of 1 to 2 fringe and did not satisfy the above standard. In the case of Related art No. 2, it had power of 2 to 4 fringes and irregularity of 1 to 1.5 fringes and did not satisfy the above standard.
Furthermore, even under the same molding condition, the obtained surface precision might be different depending on the dimension of the glass molding material or the shape of the targeted molded article. Particularly, when the glass optical element relatively large whose outer diameter is 15 mm or more is molded, the method frequently failed to obtain a desired surface precision.